Refractive power in the eye is contributed primarily by two visual components - the cornea, which provides a large fixed refractive contribution, and the crystalline lens, which provides a smaller variable contribution that allows focus from infinity to near points. This process - accommodation - results from a carefully controlled change in crystalline lens thickness, sharpness of anterior and posterior curvatures, and distance between the cornea and anterior lens surface. With increasing age, the amplitude of the accommodative response gradually decreases, resulting in the need for optical prostheses for near vision. The long-range goal of this work is to characterize fully the human accommodative mechanism and the factor(s) leading to age-dependent accommodative loss, using data obtained non-invasively from human subjects. Previous non-invasive studies of accommodation and accommodative loss in humans have led to the characterization of this process as functions of age and accommodative state, and the development of a mathematical model that both describes the process and makes predictions that can be tested experimentally. This model will be used in the next funding period to characterize fully both accommodation and the factors leading to accommodative loss, utilizing lens shape information from Scheimpflug slit-lamp photography, anterior segment and globe parameters from standard ophthalmological techniques such as pachymetry and A-scan ultrasonography, and viscoelastic data obtained from studies of post-mortem human lenses. This work will be extended by the initiation of new studies on the dynamics of accommodation in youthful and adult eyes, using a newly developed video-Scheimpflug technique; the rate of accommodation and disaccommodation will be characterized as a function of accommodative task and subject age, and correlated both with static (equilibrium) accommodation processes and with the viscoelastic properties of the lens itself. Finally, the recent characterization of human lens development in youthful eyes indicates that the accommodation mechanism may differ significantly at ages less than 18 yr; static and dynamic comparisons between data obtained from youths and adults will allow the full characterization of accommodation and the development of presbyopia in the human eye.